The Collaborative International Dictionary of English v.0.48:

Diphtheria \Diph*the"ri*a\, n. [NL., fr. Gr. ? leather (hence
   taken in the sense of membrane): cf. ? to make soft, L.
   depsere to knead.] (Med.)
   A very dangerous contagious disease in which the air
   passages, and especially the throat, become coated with a
   false membrane, produced by the solidification of an
   inflammatory exudation. Cf. Group. Diphtherial

WordNet (r) 3.0 (2006):

diphtheria
    n 1: acute contagious infection caused by the bacterium
         Corynebacterium diphtheriae; marked by the formation of a
         false membrane in the throat and other air passages causing
         difficulty in breathing